1. Field of the Invention
This invention relates to an electron scanning apparatus suitable for line analysis of specimens.
2. Description of the Prior Art
Recently, with the progress of the techniques for electronic applications, methods of analyzing a great variety of specimens are being established, a typical one of which is the line analysis technique described below.
Generally, the purpose of line analysis of a specimen is to analyze the component elements of the specimen in a given straight line on the specimen by the use of an electron scanning apparatus, for example, a scanning electron microscope equipped with an X-ray detector or an X-ray analytical instrument. There are three modes of analysis of the surface of a specimen including the plane analysis, spot analysis and line analysis. The plane analysis is for analyzing the two-dimension distribution of the elements in the surface of the specimen and has the disadvantage of a low S/N ratio. The spot analysis is concerned with the analysis of the component elements at a given point of the specimen surface and suitable for qualitative as well as quantitative analysis due to its high S/N ratio, even though it has the disadvantage that the analysis of the specimen surface is limited to a point. The line analysis, as mentioned already, is the process for analyzing the elements contained in a given straight line in the surface of the specimen and constitutes a sufficiently effective method of analysis to compensate for the disadvantages of the preceding two methods.
In X-ray analysis of a specimen, a finely focused primary electron beam is radiated on the surface of the specimen and an X-ray emitted from the radiated part of the surface of the specimen is detected by an X-ray detector. There are two methods of line analysis, mechanical and electrical. The mechanical method is one in which the radiating position of the primary electron beam is fixed while moving the specimen mechanically. In actual practice, a specimem-inching device driven by a motor or the like is used to feed the specimen in the direction of X or Y axis. The electrical method of line analysis, by contrast, consists in fixing the specimen while changing the position of the primary electron beam radiation on the surface of the specimen by linear deflection of the primary electron beam.
These methods have both advantages and disadvantages and which method is to be used is depending on the kind of specimen to be observed and the object of analysis. In the former method, that is, the mechanical method, a long linear section on the surface of the specimen is capable of being subjected to line analysis (for example, about 5 mm in an embodiment), but the movable parts of its specimen-inching device require a very high mechanical precision. On the other hand, the latter method is suitable for the case in which the line to be analyzed is not so long as in the former method, for example, the case where the length of about 0.5 mm is involved, due to the fact that changes in the radiating position of the primary electron beam causes variations in spectral conditions of the X ray. This method has the disadvantage that the result of analysis is often adversely affected by the drift of the primary electron beam, mechanical displacement of the specimen and charged condition of the specimen.
Prior to describing the present invention relating to an improvement of the former method, some problems which are encountered in the conventional method of line analysis will be described. There are two types of instruments capable of X-ray analysis of a specimen. One is the X-ray analytical instrument exclusively used for X-ray analysis of the specimen, and the other is a scanning electron microscope for observing an enlarged image of the specimen and which is equipped with an X-ray detector. Both of the above-mentioned instruments are equipped with an optical microscope either fixedly or removably. The optical microscope is provided for the purpose of determination of the point to be subjected to line analysis or observing, during the process of line analysis, the point being analyzed.
On the other hand, these apparatus are provided with such a display unit as a cathode ray tube, in addition to the optical microscope, for displaying a secondary electron image, that is, an enlarged image of the surface of the specimen. The line analysis, therefore, inevitably depends on such an optical microscope for the determination of the position of the specimen surface to be analyzed and for identification of such a position during the analytical operation. However, in the case of a scanning electron microscope equipped with an optical microscope replaceably, for example, it is necessary to reequip the electron microscope with the optical microscope if the position of analysis of the specimen is to be identified during the analytical operation. There are even some apparatuses incapable of identifying the position of analysis as it is impossible to equip it with the optical microscope in the course of analysis. Especially, a scanning electron microscope, which is capable of observation and X-ray line analysis of the surface structure of a specimen at the same time, requires no separate X-ray analytical instrument exclusively used for X-ray analysis, resulting in a great saving of analysis time. Partly due to this fact, a demand has always existed for development of a novel apparatus capable of observation and identification of the position of analysis during the X-ray analysis without any optical microscope having an only purpose of determining the position of analysis on the specimen.